This invention relates to electrodes for use in cylindrical electrochemical cells at low temperature.
The problem concerns the low coulombic efficiency of conventional zinc electrodes when they are discharged at low temperatures. In the case of the alkaline zinc/manganese dioxide battery, for example, the duration of discharge at low temperatures is limited by a rapid passivation of the zinc electrode. This passivation is caused by a building up during discharge at low temperatures of a dense layer of zinc oxide on the surface of planar zinc electrodes or within the pores or porous electrodes.
U.S. Pat. No. 3,205,097 which issued on Sept. 7, 1965 to R. R. Clune et al., describes a zinc electrode for medium low temperature operation comprised of a corrugated strip of zinc, the corrugations being deep enough to accommodate the zinc oxide formed during discharge. Usefulness of the electrode is limited to temperatures above -20.degree. F.
A variety of methods for making porous zinc electrodes with improved characteristics at -20.degree. C. have been described in the literature. It has been found however, that the major source of passivation is the formation of insoluble zinc oxide within the electrode pores. Because the solubility of the zinc oxide in the electrolyte decreases with decreasing temperature, porous electrodes have limited capacity at low temperatures. This source of passivation also adversely affects the discharge at low temperatures of electrodes prepared by compression molding of zinc fibers and needles.
In applicant's co-pending U.S. application, now U.S. Pat. No. 4,175,168, a zinc electrode prepared by folding in a zigzag fashion a strip of expanded or woven zinc to which had been attached narrow spacer bars also of expanded or woven zinc, was developed recently for rectangular (prismatic) cells. However, such a design is not suitable for use in cyclindrical cells, the configuration in which alkaline zinc/manganese dioxide cells are usually manufactured. In conventional cylindrical cells, the anode and cathode are separated by insulating battery separators. However, no means is provided for maintaining substantially uniform spacing of adjacent layers of a coiled anode.